There is a heretofore known multi-layer piezoelectric element including: a stacked body including piezoelectric layers and internal electrode layers, these layers being alternately stacked; an external electrode layer attached to a side surface of the stacked body, the external electrode layer being elongated in a stacking direction of the stacked body, the external electrode layer being connected with ends of the internal electrode layers which are exposed on the side surface; and an external electrode plate bonded to the external electrode layer therealong by an electrically conducting bonding material.
The multi-layer piezoelectric element is obtained by printing an electrically conducting paste constituting internal electrode layers on to ceramic green sheets; preparing a stacked compact by stacking the ceramic green sheets bearing the electrically conducting paste on top of each other; firing the stacked compact and subjecting it machining process such as grinding process; preparing an external electrode layer; and bonding an external electrode plate by an electrically conducting bonding material.
In this example, it has been suggested that resin silver having a composition of 80% silver and 20% epoxy-based resin is used as the electrically conducting bonding material, and a metal plate provided with slits or a metal plate formed with holes is used as the external electrode plate (refer to Patent Literature 1).
In Patent Literature 1, as the metal plate provided with slits, there is described a metal plate provided with a plurality of slits that are alternately aligned in parallel with the piezoelectric layer and the internal electrode layer. The slits extend from the opposite long sides of the metal plate toward the center thereof in such a manner that tips of the respective slits overlap each other alternately as seen in the stacking direction of the stacked body. The examples of the slit described therein include slits of various shapes such as a round shape, a triangular shape, and a rectangular shape, a slit having the form of a combination of the different shapes, and a slit with a circularly-shaped interior. Moreover, as the metal plate formed with holes, there is described a metal plate provided with round holes, rectangular holes, hexagonal holes, elliptical holes, oblique holes, and so forth.